Method and apparatus for looping back a current state to resume a memory built-in self-test

ABSTRACT

An arrangement and method of performing a memory built-in self test (MBIST) on a memory loops together the registers that store the MBIST results. When an error is detected during the MBIST by an MBIST controller, the MBIST results are shifted serially through the register loop as well as off the chip performing the MBIST. Once all the MBIST results are shifted out for further analysis by a host computer, for example, the MBIST controller is in its original state at the time the error was detected as the MBIST results are back in the registers they occupied prior to the shifting out of the MBIST results off the chip.

RELATED APPLICATIONS

This application contains subject matter related to the subject matter disclosed in copending U.S. Provisional Application Ser. No. 60/173,484, filed on Dec. 29, 1999.

FIELD OF THE INVENTION

The present invention relates to the field of network communications, and more particularly, to the testing of a memory of a network interface controller with a memory built-in self test (MBIST).

BACKGROUND OF THE INVENTION

Local area networks use a network cable or other media to link stations on the network. Each local area network architecture uses a media access controller (MAC) enabling network interface cards at each station to share access to the media.

Conventional local area network architectures use media access controllers operating according to half-duplex or full-duplex Ethernet (ANSI/IEEE standard 802.3) protocol using a described network medium, such as 10 BASE-T. Newer operating systems require that a network station be able to detect the presence of the network. In an Ethernet 10 BASE-T environment, the network is detected by the transmission of a line pulse by the physical layer (PHY) transceiver. The periodic link pulse on the 10 BASE-T media is detected by a PHY receiver, which determines the presence of another network station transmitting on the network medium based on the detection of the periodic link pulses. Hence, a PHY transceiver at station A is able to detect the presence of station B, without the transmission or reception of data packets, by the reception of link pulses on the 10 BASE-T medium from the PHY transmitter at station B.

Architectures have been developed enabling computers to be linked together using conventional twisted pair telephone lines instead of established local area network media such as 10 BASE-T. Such an arrangement, referred herein as a home network environment, provides the advantage that existing telephone wiring in a home may be used to implement a home network environment. However, telephone lines are inherently noisy due to spurious noise caused by electrical devices in the home, for example dimmer switches, transformers of home appliances, etc. In addition, the twisted pair telephone lines suffer turn-on transients due to on-hook and off-hook and noise pulses from the standard POTS telephones, and electrical systems such as heating and air conditioning systems, etc.

It is therefore important for a MAC to be informed of the conditions existing on a network at any time, and this is especially true in home network architectures. The status information is normally stored by the network controller in an external memory. In addition to the status information, the external memory also stores frame data and control information. The external memory, along with the PC board traces and connections, as well as parts of the logic within the network controller, comprise a memory subsystem. Failures in the memory subsystem may result from electrical or mechanical failure of any of the elements of the system and/or errors in the design of the PC board such as excessive loading or trace length.

In many networking and other products, embedded memory built-in self test (MBIST) circuits are used to test internal static random access memories (SRAMs) at speed. The MBIST is normally a simple circuit that reads or writes one memory location during each access. Recently, external RAM is being used to store the large amount of data required in modem networking applications. A number of different types of external memories, which allow burst mode capability, are being employed. These include pipeline burst SRAM, No Bus Latency (NoBL) SRAM and Zero Byte Turnaround (ZBT) SRAM.

In MBIST design, the user needs to learn any failing states in the MBIST. The information typically provided to users are the failing address and the failing locations in the memory. The user is notified of this information by shifting of the failing address and the failing locations out of a chip serially by using a single output pin. A single pin is used to minimize the pin count devoted to the MBIST and reduce costs. However, it is desirable to resume the test sequence from where it left off once an error is detected, in order to find other failing locations.

In conventional arrangements the failing address and the failing locations are stored, in addition to the operating registers, in an additional, parallel set of registers prior to the serial shifting of the MBIST results out of the chip. When the MBIST results are shifted out of the parallel set of registers and out of the chip, the primary registers will contain the information needed to resume the test sequence from the point at which the MBIST failed. Hence, the current state has been preserved.

The use of a parallel set of registers to preserve the current state at an MBIST failure to allow resumption of an MBIST following the shifting out of the failing information, is an inefficient use of chip real estate. This inefficiency is more acutely felt when the relative infrequency of the performance of an MBIST is considered.

SUMMARY OF THE INVENTION

There is a need for an arrangement and method that provides MBIST results from a chip, utilizing a minimal pin count, while also preserving the current state of the MBIST results to allow the original state to be restored, with minimal adverse impact on the chip real estate.

This and other needs are met by the present invention which provides an arrangement for performing a memory built-in self test (MBIST) on a memory and shifting the MBIST results out of a circuit, comprising an MBIST controller configured to perform a MBIST on a memory and generate MBIST results and a plurality of registers storing the MBIST results. The registers are connected together in a loop, with each register storing a different portion of the MBIST results. The arrangement includes an MBIST controller output coupled to an output of one of the registers in the register loop and at which the MBIST results are serially shifted out of the MBIST controller. Output control logic is provided that is configured to serially shift the different portions of the MBIST results through the loop of registers and to the MBIST controller output. This serial shifting is performed such that when the MBIST results are completely serially shifted out of the MBIST controller output, the different portions are returned to the same respective registers in which the different portions were stored prior to being serially shifted through the loop of registers.

The configuration of the plurality of register as a loop allows the MBIST results to be shifted out in a serial manner, but does not require the use of an additional set of parallel registers to restore the original state, thereby saving chip real estate. Instead, the looping back of the MBIST results through the registers restores each of the registers to its original state when the MBIST was stopped due to a test failure.

The earlier stated needs are also met by another embodiment of the present invention which provides a method of performing and shifting out the results of a memory built-in self test (MBIST) from a MBIST controller, comprising the steps of performing an MBIST on a memory, storing the results of the MBIST in a plurality of registers that are connected together to form a loop, with an output of the MBIST controller connected to an output of one of the registers, and serially shifting the MBIST results around the loop of registers and also out of the MBIST controller such that when the MBIST results are completely shifted out of the MBIST controller the registers are returned to a same state as before the shifting of the MBIST results through the registers.

The foregoing and other features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a network interface controller constructed in accordance with an embodiment of the present invention, with a network interface controller being coupled to a CPU and an external memory.

FIG. 2 is a block diagram depicting in block diagram form certain elements of the network interface controller and the external memory in order to highlight certain features of the invention.

FIG. 3 is a schematic depiction of an MBIST register in accordance with an embodiment of the present invention.

FIG. 4 is a flow chart of an exemplary method of performing an MBIST in accordance with an embodiment of the present invention.

FIG. 5 is an exemplary timing diagram of a back-to-back burst read access of a pipeline burst RAM during a MBIST in accordance with embodiments of the present invention.

FIG. 6 is an exemplary timing diagram of a back-to-back burst read access of a Zero Byte Turnaround RAM during a MBIST in accordance with embodiments of the present invention.

FIG. 7 is an exemplary timing diagram of a single memory location access of memory internal to the network interface controller while the address is ascending, in accordance with embodiments of the present invention.

FIG. 8 is an exemplary timing diagram of a single memory location access of memory internal to the network interface controller while the address is descending, in accordance with embodiments of the present invention.

FIG. 9 is a block diagram of an MBIST controller arrangement constructed in accordance with an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention addresses the problem, among others, of restoring the state of an MBIST controller to its original state at the time of a failure during an MBIST, while allowing for the serial shifting out of the MBIST results out of the chip. This is accomplished by looping together a plurality of registers that are used to store the MBIST results. These registers are connected to the MBIST controller output. When an MIBST failure is determined, the MBIST results are shifted through the register loop and also out of the MBIST controller output. When all of the results are shifted out, the MBIST results are back in their original registers, owing to the loop configuration.

Prior to describing an exemplary arrangement of the MBIST controller, a network interface controller arrangement will be described. The MBIST controller arrangement of the present invention finds utility in such an arrangement, although its applicability is not limited to such an arrangement.

FIG. 1 is a block diagram of an exemplary embodiment of a network interface controller constructed in accordance with an embodiment of the present invention. The network interface controller is described in the context of a IEEE 802.3 compliant network interface configured for sending and receiving data packets between a system memory and a network medium using established media access control (MAC) and physical layer (PHY) protocols.

The network interface 10, preferably a single-chip, 32-bit Ethernet controller, provides an interface between a local bus 12 of a computer, a peripheral component interconnect (PCI) local bus, for example, and an Ethernet-based media 50. The reference numeral 50 identifies either an actual network medium, or alternately a signal path (e.g., a media independent interface (MII)) to a physical layer transceiver coupled to the network media.

The network interface 10 includes PCI bus interface unit 16, and memory control unit 18, a network interface portion 20, a descriptor management unit 22 and register control and status unit 24. The network interface portion 20 includes an IEEE 802.3 compliant and full-duplex capable media access control (MAC) core 26, a media independent interface (MII) port 28 for connecting external 10 megabit per second, 100 megabit per second, or 1000 megabit per second transceivers, an external address detection interface (EADI) port 30, and a network port manager unit 32. The network interface 10 also includes an EEPROM interface 34 for reading from and writing to an external EEPROM, on LED control 36, and IEEE 1149.1-compliant JTAG boundary scan test access port interface 38, a clock generation unit 40, and an expansion bus 42. The expansion bus interface unit 42 interfaces to an external data memory 60 for frame data storage, control data storage and status data storage. The expansion interface unit 42 also interfaces to non-volatile (e.g., EEPROM or Flash memory) storage for boot ROM used during start-up.

The PCI bus interface unit 16, compliant with the PCI local bus specification (revision 2.2), receives data frames from a host computer memory via PCI bus 12. The PCI bus interface unit 16, under the control of the descriptor management unit 22, receives transfers from the host computer via the PCI bus 12. For example, transmit data received from the PCI bus interface unit 16 is passed to the memory control unit 18 which stores it in the data memory. Subsequently, the memory control unit 18 retrieves the transmit data from the data memory and passes it to the MAC 26 for eventual transmission to the network. Similarly, received data from the network 50 is processed by the MAC 26 and passed to the memory control unit 18 for storage in the data memory 60. Subsequently, the memory control unit 18 receives the data from the data memory 16 passes it to the PCI bus interface unit 16 for a transfer to the host computer 62 via the PCI bus 12.

The descriptor management unit 22 manages the transfers of data to and from the host computer 62 via the PCI bus interface unit 16. Data structure contained the memory of the host computer 62 specified the size and location of the data buffers along with various control in status information. The descriptor management unit 22 interfaces with the memory control unit 18 to insert control information into the transmitted data stream and to retrieve status information from the received data stream. The network interface portion 20 includes the network port manager 32 that perform auto-negotiation functions my communicating via the media 50 with a corresponding auto-negotiation unit in the link partner (e.g., the centralized hub, repeater, work station, or switch).

The network interface 10 also includes a power management unit 44 that enables remote activation (i.e., turn-on) of the host computer 62 via the network medium 50 by detecting a predetermined pattern on the medium 50 according to Microsoft On Now and ACPI specifications, including compliance with Magic Packet technology and PCI bus power management interface specification protocols.

The network interface 10 also includes a MIB counter unit 46 which excepts information from the MAC 26 regarding frame transmission and reception and maintains the statistics necessary for network management. The statistics are accessed by the host computer 62 via the PCI bus interface unit 16.

FIG. 2 depicts the network interface controller and external memory 60. Only certain elements of the network interface controller 10 are depicted for purposes of the following description.

The network interface controller 10 uses the external memory 60 for receive and transmit status. The size of the memory can be up to 4 Megabytes, in preferred embodiments, organized as 1M×32 bits. The memory size is larger in other embodiments of the invention. The size of the memory 60 is indicated by the contents of an external memory size register 66. The external memory size is loaded into the external memory size register 66 from an EEPROM interface 34.

The external memory 60 is programmed in units of 512-byte pages. To specify how much of the external memory 60 is allocated to transmit and how much is allocated to receive, a user should program a boundary register (not depicted) with the page boundary where the receive buffer begins. The boundary register is also programmed in units of 512-byte pages. The transmit buffer space starts at the first address in the boundary register. It is up to the user or a software driver to split up the memory for a transmit or receive, and there is no defaulted value in preferred embodiments. The minimum memory size required in preferred embodiments is for 512-byte pages for each transmit and receive queue which limits the memory size to be at least 4 Kbytes.

The network interface controller 10 does not allow software diagnostic access to the external memory 60. However, the network interface controller 10 provides software access to an internal memory built-in self-test (MBIST) controller (i.e. control logic) 68 which runs extensive at-speed tests on the external memory 60, the internal access logic 18 for the memory 60, and PC board interconnect.

The expansion bus interface 42 (external memory interface) includes external memory address pins [19:0] that provide addresses for both the external memory 60 and an external boot ROM device. All of these outputs [19:0] are forced to a constant level to conserve power while no access on the external memory bus is being performed. The ERD [31:0] pins provide data bits [31:0] for external memory 60 accesses. These signals are forced to a constant level to conserve power while no access on the external memory bus is being performed. For boot ROM accesses, the ERD [7:0] pins provide the data bits [7:0] for boot ROM accesses.

{overscore (ERCE)} serves as the chip enable signal for the external memory 60. This is asserted low when the external memory address on the ERA [19:0] pins is valid.

{overscore (FLCS)} serves as the chip select for the boot device. It is asserted low when the boot ROM address and ERA [19:0] pins is valid.

{overscore (EROE)} is the external memory output enable signal that is asserted active low during external memory device read operations to allow the external memory 60 to drive the ERD [31:0] data bus. It is de-asserted at all other times.

{overscore (ERWE)} is the external memory byte enable that provides the write enable for write accesses to the external memory 60 in the Flash (boot ROM) device.

{overscore (ERADSP)} is the external memory address strobe signal that provides the address strobe signal to load the address into the external memory 60.

{overscore (ERADV)} uses the external memory address advance that provides the address advance signal to the external memory 60. This signal is asserted low during a burst access to increment the address counter in the external memory 60.

ERCLK is the reference clock for all synchronous memory accesses.

Referring to FIG. 2, the network interface controller 10 of the present invention employs control logic 68 which controls the MBIST procedure to determine the size of the external memory 60 and verify its correct operation. Before describing the adaptive features of the present invention to accommodate different external memories, the MBIST itself according to embodiments of the invention will be described. The control logic 68 may be part of the memory controller 18 or logically separate from this controller. The control logic 68 employs the MBIST procedure to exercise the memory subsystem. The memory subsystem includes the external memory 60, the input/output logic (the expansion bus interface 42), and the board traces in the external memory 60. This test can run at normal memory operating speed and test normal and/or worst-case access patterns.

The MBIST is a pattern generation test that comprises writing data to the external memory 60 and reading this data from the external memory 60 and comparing it with expected data. Typically, such a MBIST is found in tightly coupled MBIST logic that tests for fabrication errors. The term “tightly coupled” is meant to indicate that the logic is located physically within the RAM block and has access to internal RAM signals that are not usually accessible via pins on the edges of a RAM chip. By contrast, the present invention provides MBIST logic on the network interface controller 10, which connects to an external memory 60, and is thus not tightly coupled. This allows different sized external memories 60 to be used in accordance with the requirements of the user.

The controller logic 68 employs the MBIST register 64 and an SRAM size register 66 to perform the size determination and verify the operation of the external memory 60. These registers may form part of the register control and status unit 24. A schematic depiction of the MBIST register 64 is provided in FIG. 3. The MBIST register 64, in preferred embodiments, is a 64 bit register. A description of the bit fields in the register now follows.

Bit 63 is the DM_DONE bit which indicates whether the MBIST is done. This bit is set to 1 when the automatic memory test has stopped, either because the test has completed or because an error was detected. It is cleared to 0 when either a DM_START or DM_RESUME is set. The DM_DONE bit is read-only.

Bit 62 is the DM_ERROR bit, which acts as the MBIST error indicator. This bit is set to 1 when the memory test logic has detected a memory error. It is cleared to 0 when either DM_START or DM_RESUME is set. This bit is also read-only.

DM_START is the MBIST start bit. Setting this bit to 1 resets the MBIST logic, including the DM_ERROR and DM_TEST_FAIL bits, and starts the memory test process. DM_START should not be set at the same time that the DM_RESUME bit is set. When the memory test stops, the DM_START bit is automatically cleared. This bit is read/write.

DM_RESUME bit is bit 60 and acts to resume the MBIST. Setting this bit to 1 restarts the memory test sequence at the point where it last stopped. Setting this bit clears the DM_ERROR bit, but it does not clear the DM_TEST_FAIL bit. This bit should not be set at the same time that the DM_START bit is set. The DM_RESUME bit is automatically cleared when the memory test stops. This bit is read/write.

The DM_FAIL_STOP bit is bit 59 and performs the MBIST stop on failure control. When this bit is set to 1, the memory test will stop each time an error is detected. When this bit is cleared to 0, the memory test will run to completion, regardless of the number of errors that are detected. This bit is read/write.

Bit number 58 is the DM_TEST_FAIL bit that acts as the MBIST test failure indicator. This bit is set when a memory test error is detected. It is reset when DM_START is set to 1. It is not cleared when DM_RESUME is set to 1. This bit is read only.

Bit number 57 is reserved, and written as 0, read as undefined.

DM_DIR bit (bit 56) is the MBIST test direction. This bit is set to 1 when the MBIST memory pointer was counting down when the test stopped. It is cleared to zero when the MBIST memory pointer was counting up when the test stopped. This bit is read only.

Bit 55-54 represent the DM_FAIL_STATE bits and acts as the MBIST error offset indicator. This field indicates offset of the location of the last memory test error with respect to the value of the DM_ADDR field. Interpretation of this field depends on the value of the DM_DIR field which indicates whether the address pointer was counting up or down when the error was detected. This field is read only. An exemplary assignment of the error location as a function of the DM_DIR bit and the DM_FAIL_STATE bits is provided below.

DM_DIR FAIL_STATE Error Location 0 00 Error at DM_ADDRESS 0 01 Error at DM_ADDRESS−1 0 10 Error at DM_ADDRESS−2 0 11 Error at DM_ADDRESS−3 1 00 Error at DM_ADDRESS 1 01 Error at DM_ADDRESS+1 1 10 Error at DM_ADDRESS+2 1 11 Error at DM_ADDRESS+3

The DM_BACKG bits 53-52 form the MBIST background bits. This field contains the background pattern that the memory test logic was using when the test stopped. This field is read-only.

Bits 51-32 are the DM_ADDR bits and form the MBIST address. This field contains the contents of the MBIST address pointer at the time of the test stop. Because of the pipelined nature of the external memory 60, this value may not be the location of the memory error. The actual error location is obtained by adding or subtracting the contents of the DM_FAIL_STATE field as described above. This field is read-only.

The DM_DATA bits are bits 31-0. This field contains the last data that the memory test logic read from the external memory 60. If the DM_ERR and DM_FAIL_STOP bits are both set to 1, the contents of this field contains an error. This field is read only.

Although the specific assignment bits in the MBIST register are provided in FIG. 3 and the above description, this assignment of bits is exemplary only as the MBIST register 64 may be configured differently but otherwise consistent with the description of the present invention. The contents of the register are cleared to 0 when the reset pin is asserted.

FIG. 4 is a flow chart of an exemplary embodiment of the method of verifying the operation of the external memory 60 and its size, using the control logic 68 in accordance with embodiments of the present invention. After the network interface controller 10 is started, the external memory (SRAM) size is programmed into the SRAM_SIZE register 66 to a minimum allowed value of 4. After this step is completed, the procedure continues to step 72, in which the DM_START and DM_FAIL_STOP bits in the MBIST register 64 are written. For example, bits 63:56 are written with 0x28. The remainder of the MBIST register 64 ignores writes so that it may be written with arbitrary data or not written at all. This action automatically starts the memory test process by the control logic 68.

The control logic 68 reads the DM_DONE bit (bit 63) and the DM_ERROR bit (bit 62) until the DM_DONE bit 63 is set to 1. This is depicted in FIG. 4 as steps 74 and 76. Once the DM_DONE bit is set, as determined in step 76, it is then determined in step 78 whether the DM_ERROR bit is set. If the DM_ERROR bit is set, the external memory 60 is considered defective, the error is reported in step 80 and the memory test process is exited.

When the DM_DONE bit is set, as determined in step 76, but the DM_ERROR bit is not set as determined in step 78, then the SRAM_SIZE register 66 is reprogrammed to the maximum value 0x8000, as depicted in step 82. Steps 74 and 76 are then re-performed as steps 84 and 86. In other words, the DM_START and DM_FAIL_STOP bits are written in the MBIST register 64 by the control logic 68. The memory test is then performed again until the DM_DONE bit is set, with the control logic 68 reading the DM_DONE and DM_ERROR bits. The process loops back to step 84 if the DM_DONE bit is not set as determined in step 86.

When the memory test is completed, indicated by the DM_DONE being set that is determined in step 86, it is again determined whether the DM_ERROR bit is set. If the DM_ERROR is 0, the current value in the SRAM_SIZE register 66 is reported out to the CPU 62 as the size of the external memory (step 92). The process is then exited. If the DM_ERROR bit is not set (i.e. not equal 0), the SRAM_SIZE register 66 is programmed to ½ the maximum (0x4000) and steps 84-86 are repeated. This process of reprogramming the SRAM_SIZE and performing the memory test until the DM_ERROR bit is 0, is repeated using the binary search algorithm until the size of the external memory 60 is determined.

The above-described MBIST provided by the MBIST controller 68 is employed regardless of the type of external memory 60 coupled to the external memory interface 42. The present invention employs adaptive logic 69 in the MIBST controller 68 to determine the actual failing address of an MBIST independent of the external memory type.

The concern regarding different type external memories, in conjunction with testing these memories with a single MBIST, is relieved by providing a fail state indicator used by the adaptive logic 69 to accurately determine the memory location of a MBIST failure. FIG. 5 depicts a back-to-back burst read access of an external memory of a pipeline burst SRAM, and FIG. 6 depicts a back-to-back burst read access of a zero byte turnaround SRAM.

A comparison of the exemplary timing diagrams of FIGS. 5 and 6 reveals the concern regarding using a single MBIST with different external memory types. Due to the latency required when performing back-to-back operation in a pipeline burst SRAM, the MBIST counter does not increment at clock 6, for example (FIG. 5). By comparison, the zero byte turnaround SRAM has no such latency constraint (FIG. 6). Accordingly, the MBIST controller 68, when performing the same MBIST for the different memory types, would have difficulty in pinpointing the exact location of the failed memory location.

As seen in FIG. 5, a back-to-back burst read access operation is being performed. In the pipeline burst SRAM, assume that memory location 0002 is the location to be tested. The data will only be available at clock 5. If the data being read is wrong, the MBIST controller 68 flags an error at clock 6. The address counter of the MBIST at clock 6, as seen in FIG. 5, is registered with address 0004. However, this address (0004) is not the actual memory location of the failure. The actual memory location is 0002, or 2 locations less than that registered with the MBIST address counter.

As another example, using the same FIG. 5, if the memory location of the failure is 0001, the error would be flagged at clock 5. The MBIST address counter would also be registered with memory location 0004 at clock 5, as it was in the previous example at clock 6. In this case, though, the actual memory location of the failure is 0001, which is 3 locations less than that registered with the MBIST address counter.

These examples demonstrate that the one clock latency of the pipeline burst SRAM when performing back-to-back operation causes the MBIST controller 68 difficulty in determining the actual failing location, as it is not consistently offset from the location registered with the MBIST address counter when the failure is flagged. When a different type of SRAM is used, such as a zero byte turnaround SRAM, the latency concerns may be different. For example, in FIG. 6, there is no latency problem in back-to-back operation. Hence, an error at location 0002 will be reported at clock 6, registered with memory location 5. The offset of reported memory location and the actual failing memory location remains at 3 (0005-0002), throughout the back-to-back burst read access. The MBIST controller 68 therefore needs to be aware of the type of external memory 60 coupled to the external memory interface 42 of the network interface controller 10. Once properly informed of the external memory type, the MBIST controller 68 can use the same MBIST for each external memory type, but take account of the different latencies of the different types to accurately determine the actual failing memory location.

The present invention employs a fail state indicator that indicates the state of the MBIST control state machine when an error occurs, in order to determine the actual failing memory location. The fail state indication (DM_FAIL_STATE) is stored in the MIST 64, as described earlier. Also stored in the MBIST register 64 is the address direction (DM_DIR), the address location at the time the MBIST was stopped (DM_ADDR), and the last data (DM_DATA) that the MBIST controller 68 read from the external memory 60. The expected data is built from the data background (DM_BACKG). These are two bits (00, 01, 10, 11) that are duplicated, in preferred embodiments, to form the input to the external memory during the MBIST. For example, with an 8-bit data bus, the expected data can be built using the two data background bits by replicating the bits four times. This produces expected data of 00000000, 01010101, 10101010, or 11111111. This scheme of replicating the data background bits reduces the number of registers required for the MBIST.

In preferred embodiments of the invention, the address direction and the fail state indicator are used by the MBIST controller 68 to calculate the actual failing memory location. The MBIST controller 68 will then report this failing memory location to the user. During the MBIST, the value of DM_FAIL_STATE in the MBIST register 64 is changed by the MBIST control state machine in accordance with the type of external memory 60 and the current state of the memory operation. For example, by examining the SRAM_TYPE field in a configuration register, the MBIST controller 68 is aware that the external memory is a pipeline burst SRAM (assuming such a memory is coupled to the external memory interface 42). During the one clock latency between bursts (clocks 5 and 6 in FIG. 5), the MBIST control state machine changes the value of DM_FAIL_STATE in the MBIST register 64 from ‘11’ to ‘10’. This indicates, as shown in FIG. 5, that the offset to determine the actual failing memory location from the address of the memory being read when the error is flagged has changed from 3 to 2. Hence, in the example of FIG. 5, an error at location 0002 is flagged at clock 6, registered with address 0004. The value of DM_ADDR is therefore equal to 0004. The address direction (DM_DIR) is set to ‘0’, indicating that the MBIST control state machine is counting up. The value of DM_FAIL_STATE at this time is set to ‘10’, indicating an offset of −2, since DM_DIR has a value of ‘0’. (If DM_DIR was set to ‘1’, the offset would be +2).

The MBIST controller examines the MBIST register 64 when the MBIST stops and an error is detected, as discussed earlier with respect to the exemplary method of FIG. 4. The offset is obtained from the DM_FAIL_STATE and DM_DIR values, and the actual failing memory location is then calculated by the MBIST controller 68 by offsetting the value of DM_ADDR with the determined offset value.

In the example of a different type of external memory, such as a zero byte turnaround SRAM, the value of DM_FAIL_STATE may not be changed during the MBIST, as the latency concerns are not the same as in the pipeline burst SRAM. The present invention, however, provides flexibility to allow the same MBIST to be used with different types of external memories, by accounting for the different latencies of these different memory types. The invention is therefore not limited to the two different types of external memory types described above as exemplary external memories for coupling to the network interface controller, but rather, may be used with other types of external memories. The MBIST control state machine would then be required to properly set the value of the fail state indicator.

The implementation of the 2-bit fail state indicator of the present invention can be readily incorporated into existing MBIST state machines. This is shown in a following partial Verilog HDL code which shows the burst access mode of the implementation. The states BR1, BR2, BR3, and BR4 rpresent the burst read of first, second, third and fourth memory locations respectively. As the data is available to be read two clocks later, the fail-state signal is latched twice (e.g., through to D flip-flops) before updating the scan out information in the MBIST register 64.

BR1: begin   NEXT_STATE=BR2   fail-state = 2′b11;   ... end BR2: begin   NEXT_STATE=BR3   fail-state = 2′b11;   ... end BR3: begin   NEXT_STATE=BR4   if (ZBT) fail-state = 2′b11 else   fail-state = 2′b10;   ... end BR4: begin   NEXT_STATE=BR5   if (ZBT) fail-state = 2′b11 else   fail-state = 2′b10;   ... end ... always@(posedge CLK) begin   fail_state1<= fail_state;   FAIL_STATE <= fail_state1 end

FIGS. 7 and 8 are exemplary timing diagrams of a single memory location access of internal SRAM on the network interface controller 10. This internal SRAM may be part of the memory control unit 18, or logically separate therefrom. In FIG. 7, the address is ascending during the single memory location access, while the address is descending in FIG. 8. In the example of FIG. 7, with the address ascending (DM_DIR=‘0’), the actual failed memory location is calculated as 0001−1=0, where 0001 is the value of DM_ADDR and ‘0’ is the actual failing memory location. The offset of ‘−1’ is determined from the value of DM_FAIL_STATE (‘01’), which indicates an offset of 1, and DM_DIR (‘0’), which indicates that the offset should be subtracted from the value of DM_ADDR.

In FIG. 8, the address is descending, so that DM_DIR will have a value of ‘1’. The value of DM_ADDR for the error in FIG. 8 is FFFE. Because the addresses are descending (see ADD in FIG. 8), the address offset of 1 (DM_FAIL_STATE=‘01’) is added to the value of DM_ADDR to produce the actual failing memory location. In this example, the actual failing memory location is then FFFE+1=FFFF. This value is reported out to the user by the MBIST controller 68.

FIG. 9 depicts in more detail an exemplary embodiment of the MBIST controller 68 in accordance with the present invention. The MBIST controller 68 includes two sections: MBIST control logic 100 and an output analyzer 102. The MBIST control logic 100 generates a memory address as well as the failing address when an error or mismatch is found. The output analyzer 102 compares the expected data read from the memory (internal or external) and generates the failing memory locations.

The output analyzer 102 receives the read data from the memory under test and compares it to the data background (DK_BACKG) supplied to it by the MBIST control logic 100. This data background formed the data that was written into the memory under test by the MBIST control logic 100. The output analyzer 102 also receives a compare or invert compare signal from the MBIST control logic 100 to control the comparison procedure.

When the output analyzer 102 detects a difference between the data background (i.e., the expected data) and the data actually read from the memory, an error is indicated. The output analyzer 102 generates a shift enable signal that stops the MBIST control logic 100 from continuing the MBIST on the memory under test and begins the process of scanning out data from the MBIST controller 68.

The MBIST control logic 100 and the output analyzer 102 contain a number of registers 106 that are coupled together in a serial, loop arrangement 104. This loop 104 allows the data to be scanned out serially from a single pin, while also allowing the original state at the time of the error detection to be restored.

For illustrative purposes, the MBIST control logic 100 is shown in FIG. 9 as having three of the registers 106 in the loop 104, and the output analyzer 102 is depicted as having two of the registers 106 in the loop 104. This is exemplary only, as the number of registers 106 may be greater or less than this number in accordance with the present invention. Also, the physical location of the registers 106 does not have to be within the MBIST control logic 100 and the output analyzer 102. Rather, their physical location may be within the register control and status unit 24, depicted in FIG. 1, or elsewhere on the chip.

The output of the last register 106 in the MBIST control logic 100 is coupled to an MBIST controller output 108 that is routed to a single pin where the data can be serially shifted off the network interface controller 10. The output of the last register 106 in the MBIST control logic 100 is coupled to the input of the first register 106 in the output analyzer 102. The output of the last register 106 in the output analyzer 102 is coupled to the input of the first register 106 in the MBIST control logic 100.

The thus formed loop 104 allows the data contained in the registers 106 to be serially shifted from register 106 to register 106, as well as out the MBIST controller output 108. The sequence of scan out information includes the address direction, which is one bit that indicates whether the addresses are counting up or counting down. The scan out information includes the RAM data, which is the output from the memory (external or internal) that is currently under test. The bit count for the RAM data is 32 bits. The next data in the sequence that is scanned out is the fail state information, which is two bits that indicate the state of the MBIST control state machine run by the MBIST control logic 100, as described earlier. The next data in the sequence are the twenty bits that form the address of memory being read when the error occurs. The final data in the scan out sequence are the two data background bits which form the basis for the expected data used to compare with the actual data read from the memory.

The data above are present in their respective registers 106, formed by flip-flops, for example, when an error is detected by the output analyzer 102. The shift enable signal generated by the output analyzer 102 (i.e., output control logic) causes the data to serially shift through the flip-flops forming the register 106 in the loop 104 and out the MBIST controller output 108. When all of the data has been scanned out, the data will be in their original flip-flops (registers 106) due to the loop configuration of the registers. In other words, the address direction bit will have shifted through each of the flip-flops in registers 106 in the loop 104 formed in the MBIST control logic 100 and the ouptut analyzer 102 during the scan out operation following an error, until all of the data has been scanned out. At that point, the address direction bit will have been returned to the flip-flop it originally occupied before the scanning out of the data. The same is true for the other data.

Once the data has been scanned out of the MBIST controller 68, a host computer may read this information, from the MBIST access register 64, for example, assuming that the register is readable through a normal PCI read. The host computer is able to examine this information to determine the actual failing memory locations, in the manner described earlier, using the fail state information, the address direction, the address, etc.

Since all of the data is returned to the original registers 106 which they occupied prior to the scanning out of data following an error detection, the MBIST control logic 100 and the output analyzer 102 are restored to their original state prior to the scanning out. The MBIST process can then resume from this point as the original state has been restored.

The present invention allows the restoration of an original state after error detection in an MBIST and the scanning out of the MBIST results in a serial manner. This keeps the pin count to a minimum (one pin), but the invention does not increase the number of registers in the chip, as would parallel register configurations, since the registers are looped together.

Although the present invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being limited only by the terms of the appended claims. 

What is claimed is:
 1. An arrangement for performing a memory built-in self test (MBIST) on a memory and shifting the MBIST results out of a circuit, comprising: an MBIST controller configured to perform a MBIST on a memory by comparing data written into the memory with data read from the memory and generate MBIST results; a plurality of registers storing the MBIST results, the registers being connected together in a loop, each register storing a different portion of the MBIST results; an MBIST controller output coupled to an output of one of the registers in the register loop and at which the MBIST results are serially shifted out of the MBIST controller; output control logic configured to serially shift the different portions of the MBIST results through the loop of registers and to the MBIST controller output such that when the MBIST results are completely serially shifted out of the MBIST controller output the different portions are returned to the same respective registers in which the different portions were stored prior to being serially shifted through the loop of registers.
 2. The arrangement of claim 1, wherein the MBIST controller includes MBIST control logic and an output analyzer coupled to the MBIST control logic.
 3. The arrangement of claim 2, wherein the MBIST results include address information from which a failing memory location is determinable.
 4. The arrangement of claim 3, wherein the MBIST results include output data from a memory under test.
 5. The arrangement of claim 4, wherein the address information includes read address information which indicates an address of a memory location being read when an error is recognized during the MBIST.
 6. The arrangement of claim 5, wherein the address information further includes offset information which indicates an offset of the read address information, the offset information and the read address information being combinable to determine a failing memory location.
 7. The arrangement of claim 6, wherein the MBIST control logic includes an MBIST state machine and the offset information includes fail state information that indicates a state of the MBIST control state machine.
 8. The arrangement of claim 7, wherein the offset information further includes address direction information which indicates whether the address is ascending or descending when the an error is recognized during the MBIST.
 9. The arrangement of claim 8, wherein the output analyzer includes comparison logic to compare expected data with the output data and recognizing an error when the output data does not match the expected data.
 10. The arrangement of claim 9, wherein the plurality of registers stores the address information, the output data from a memory under test, the read address information, the fail state information, the address direction information, and the expected data information.
 11. A method of performing and shifting out the results of a memory built-in self test (MBIST) from a MBIST controller, comprising the steps of: performing an MBIST on a memory by comparing data written into the memory with data read from the memory; storing the results of the MBIST in a plurality of registers that are connected together to form a loop, with an output of the MBIST controller connected to an output of one of the registers; and serially shifting the MBIST results around the loop of registers and also out of the MBIST controller such that when the MBIST results are completely shifted out of the MBIST controller the registers are returned to a same state as before the shifting of the MBIST results through the registers.
 12. The method of claim 11, wherein the MBIST results include address information from which a failing memory location is determinable.
 13. The method of claim 12, wherein the MBIST results include output data from the memory under test.
 14. The method of claim 13, wherein the MBIST controller includes MBIST control logic and an output analyzer coupled to the MBIST control logic in a loop, each of the output analyzer and the MBIST control logic containing separate sections of the loop of registers.
 15. The method of claim 14, wherein performing an MBIST includes comparing expected data with data output from the memory and generating an error signal when the output data does not match the expected data.
 16. The method of claim 15, further comprising: halting the MBIST until the MBIST results are completely shifted out of the MBIST controller and the registers are returned to the same state as before the shifting of the MBIST results through the registers; and re-starting the MBIST to continue from the point in the MBIST where the MBIST failed. 